Modern rapid distribution systems for fresh meats rely upon refrigeration and plant sanitation to provide a wholesome product to the consumer. Although fresh meats in the United States are generally safe and free of hazardous levels of pathogens, the microbial quality may be poor; high levels of nonpathogenic spoilage bacteria, frequently present, dramatically shorten shelf life and affect the taste and appearance of the meat. In response to this problem several states and the Federal Government have adopted or proposed standards regulating the bacterial content of fresh meat.
It is recognized that refrigeration is not a complete solution to spoilage problems because the principal spoilage bacteria are psychrotrophes which grow well at 5.degree.-15.degree. C., or mesotrophes which are adapted to grow at lower temperatures. Ayres, J. C. (Food Research, 25:1, 1960) found that the dominant microorganisms growing on refrigerated beef undergoing advanced spoilage comprised the Micrococci and Pseudomonads; even at temperatures as high as 15.degree. C. the growing bacterial population was dominated by motile gram negative rods of the Pseudomonas generae.
These Pseudomonads, however, comprise only a minor portion of the initial contaminating population. This population is extremely diverse, and species of Acinetobacter, Moraxella, Flavobacterium, and Aeromonas may be present in substantial numbers (See Jay & Shelef, Food Technol., 22:186 1978), in addition to members of the Enterobacteriace. In the course of early spoilage at refrigerated temperatures, most of these species are displaced by nonproteolytic Pseudomonads. Off-odors characteristic of spoilage are detected when this sub-population grows to a level in excess of 10.sup.7 -10.sup.8 organism per square inch of meat surface.
The mechanism of displacement of some generae of bacteria by the Pseudomonads in the early stages of spoilage is not well understood. These organisms are physiologically versatile, and readily attack a wide variety of low molecular weight nitrogenous substances whose by-products of metabolism account for the off-odors associated with meat spoilage. However, it is known that attack of these substances is strongly repressed by glucose which comprises about 0.18 percent of the meat (Gill, C. O., J. Appl. Microbiol., 41:401 1976). Substantial development of the psychrotrophic Pseudomonads, Aeromonas, etc. in early spoilage may thus be preceded by the growth of other organisms which preferentially deplete surface glucose thereby relieving repression of the Pseudomonad development. As the temperature of chilling meat becomes refractory to bacterial growth generally, the Pseudomonads, etc. then emerge as the dominant population.
Bacterial contamination of meat has been the subject of extensive studies whose conclusions are here set forth. The microorganisms present in retail portions are derived principally from the initial bacterial load on the carcass surface immediately post-slaughter; thus, meat portions, such as hamburger, having high bacterial counts are traceable to carcasses having high surface contamination. (For example, see Elliot, et al., Appl. Microbiol., 9:452 1961.) The primary source of such contamination is the gut and hide of the animal itself, although packing house environment (floors, chill room, cutting room, etc.), and handling by packing house workers are all substantial sources of contamination. (Frazier, Food Micro Biology, Chapt. 16, 2ed., 1967.)
Contamination of carcasses from these sources is not uniform; initial bacterial counts vary from 10.sup.2 organisms per square inch, or less, to greater than 10.sup.6 organisms per square inch (Ingram & Roberts, "The Microbiology of the Red Meat Carcass and the Slaughterhouse," Meat Research Institute publication, Langford, Bristol, England, 1976). This variation in contamination occurs between different zones of the same carcass (including adjacent areas as small as one square inch), and among different carcasses. The relation of the magnitude and composition of initial contamination at any selected site to subsequent bacterial development at the same site has not heretofore been studied. Little is known about the kinetics of prespoilage proliferation of naturally-occurring mixed populations of such bacteria.
Several available processes eliminate bacteria from meat by killing them with a contact disinfectant(s) applied in the form of a spray to the carcass surface during chilling. U.S. Pat. No. 3,745,026 (Hansen) disclosed such a process utilizing 50-200 ppm of aqueous chlorine (hypochlorous acid). U.S. Pat. No. 4,021,585 (Svoboda, et al.) describes an alternative process utilizing 5-50 ppm of aqueous chlorine dioxide. Both processes achieve reduced bacterial counts during the chill cycle (18-24 hrs. post-slaughter) by killing bacteria introduced onto the carcass during slaughter procedures. This is shown by the reduction in viable colony-forming bacteria present at the end of the chill cycle as compared with counts at the beginning of such period after carcasses are conveyed to the chill room from the kill floor.
A major limitation of processes utilizing aqueous chlorine (hypochlorous acid and its calcium or sodium salts) is rapid regrowth of microorganisms on the treated surfaces. Several investigators have reported initial bactericidal action resulting in up to about 3 logs.sub.10 of reduction in aerobic bacterial counts (Kotula, et al., J. Anim. Sci., 39:674 1974); Bailey, C., "Spray washing of lamb carcasses," 17th European meeting of meat researchers, Bristol, England 1971.) However, others have noted substantial bacterial regrowth during the first 48 hours post-slaughter; rapid regrowth accounts for net increases after initial reductions. (For example, see Anderson, et al., J. Food Sci., 42:326 1977.)
A second major problem with such use of chlorinated contact disinfectants is reaction of the agent with meat components to produce chloro-organic derivatives such as chloro-substituted lipids, and oxidative products. These chemical derivatives may pose a health hazard, especially the class of halomethanes (known to be carcinogenic) formed by reaction of bactericidal levels of hypochlorous acid with humic or other organic substances. Reaction of chlorine dioxide at bactericidal concentrations with meat components results in low but detectable levels of organic chlorine, as noted in Cunningham & Lawrence (J. Assoc. Off. Anal. Chem., 62:482 1979).
Other agents such as inorganic and organic acids have also been applied to carcass surfaces in the form of aqueous sprays, as described in Carpenter, J. A., Proc. Meat Indust. Res. Conf., Chicago, 1972. Use of these agents has not received widespread acceptance because of reported surface damage to the carcass and off-odors and flavors imparted to the meat at bacteriostatic concentrations.